It is well known that alkyl tert-alkyl ethers can be produced by reacting a primary alcohol with a tertiary olefin from 4 to 7 carbon atoms over a suitable catalyst. Two such ethers of great commercial value as motor fuel octane enhancers are methyl t-butyl ether (MTBE) and t-amyl methyl ether (TAME). These are made by reacting isobutylene and isopentenes respectively with methanol.
Such etherification reactions are exothermic and equilibrium-limited. They are generally carried out in the liquid phase in one or two fixed bed catalytic reactors in series, and heat is removed by circulating liquid through external heat exchangers. Catalysts are generally strongly acidic ion exchange resins such as "Amberlyst 15, " commercially available from Rohm and Haas Co. Other suitable catalysts are a bifunctional catalyst which is a macroporous, strongly acidic cation exchanger with sulfonic groups and a trace of palladium or a selective zeolite catalyst such as ZSM-5 or ZSM-11.
These etherification catalysts are so selective for the tertiary olefins-methanol reactions that the diolefins present in the particular feedstock (such as butadiene and C.sub.3 -C.sub.4 acetylenes), the carbonyl compounds, and other paraffins are not reacted in the reactions.
The tertiary olefin conversions are generally limited in the range of 90 to 96% in a single reactor system using excess methanol. To achieve higher conversions, a two-stage reactor system using between 2 to 20% excess methanol is generally practiced. Excess alcohol is also beneficial to suppress polymerization of olefins to dimers and trimers. However, the excess methanol in the MTBE or TAME processes must be removed for recycle to the reactors and for high purity ether products and C.sub.4 to C.sub.7 raffinates. Unfortunately, methanol forms azeotrope mixtures with these ethers and C.sub.4 to C.sub.7 raffinates. Separation by ordinary distillation is very difficult and, as a result, both energy- and capital-intensive.
Several techniques have been disclosed in the prior art to remove methanol form etherification products.
U.S. Pat. No. 3,726,942 discloses a MTBE process in which the MTBE effluent stream is first sent to a distillation column to separate MTBE (bottom product) from C.sub.4 raffinate (overhead product). The crude MTBE product is water washed to remove methanol. The crude C.sub.4 raffinate is also water washed to remove methanol; alternatively, mol sieve is used to remove methanol. The methanol-water is separated by distillation and recycled to the MTBE reactor.
U.S. Pat. No. 3,846,088 discloses a similar process in which the crude MTBE product from the distillation column bottom is subject to water wash and then mixed with a paraffin (C.sub.5 -C.sub.10) to reject the residual water. U.S. Pat. No. 4,118,425 discloses a process in which crude MTBE/TAME from the reactor is first subject to water wash and then to distillation to produce C.sub.4 -C.sub.5 raffinate overhead and pure MTBE/TAME product bottom.
U.S. Pat. No. 4,302,298 discloses a MTBE process in which the reactor effluent mixture is fed to a distillation column to produce a MTBE bottom product. The overhead vapor is condensed and water-washed to remove methanol before refluxing; then methanol-water is separated by distillation. Similarly, U.S. Pat. No. 4,324,924 discloses a MTBE process including a water wash step after the second reactor/distillation sequence.
U.S. Pat. No. 4,334,964 discloses a MTBE/TAME process using a water wash step to recover and recycle methanol from the reactor effluent. The methanol-water separation is done in a distillation column with a side draw to remove tertiary alcohol. U.S. Pat. No. 4,544,776 discloses a similar process as above.
U.S. Pat. No. 4,409,421 discloses a process for preparing a pure tertiary olefin, in which alkanol and tertiary alkyl ether are separated by distillation followed by adsorption using synthetic ion exchange resins. In U.S. Pat. No. 4,447,653 an adsorptive separation step is used to remove methanol from overhead raffinate stream of the distillation column (with water wash); regeneration is achieved by passing hot recycle isoparaffin-rich stream throughout the adsorbent. U.S. Pat. No. 4,465,870 discloses a similar process as above, and using the adsorbent of Type 5A or 13X molecular sieves for methanol and MTBE removal from C.sub.4 raffinate.
U.S. Pat. No. 4,605,787 discloses a MTBE process which uses small pore zeolites (3A, 4A, 5A and chabozite) for methanol/MTBE separation from the crude MTBE bottom stream of the distillation column. In EP-205562 the methanol is recovered from the distillation overhead crude C.sub.4 raffinate stream) using Na cation form of zeolite (pore size is about 4A); the bed is regenerated by passing a hot C.sub.4 -C.sub.5 hydrocarbon feed stream before entering the reactor.
U.S. Pat. No. 4,218,569 teaches the use of a glycol to remove methanol.
U.S. Pat. No. 4,405,409 discloses a membrane method and apparatus for dehydrating mixtures of organic liquids and water. Example mixtures are ethanol/water, isopropanol/water, ethylacetate/water and pyridine/water. An example membrane material is modified cellulose acetate. U.S. Pat. No. 4,547,530 discloses a pervaporation membrane comprising a blend or alloy of poly-2-oxazolines and thermal plastic polymers for separating ethanol/hexane mixtures. U.S. Pat. No. 4,590,098 discloses a process for producing a composite membrane by cross-linking silicone resin with the dense layer of polyimide resin, useful for water/ethanol pervaporation separation. U.S. Pat. No. 4,591,440 claims a membrane for liquid separation, which comprises mainly poly (sub-stituted acetylene), especially for ethanol/water separation.
U.S. Pat. No. 4,570,026 discloses a process for producing high purity isobutene by MTBE decomposition by contacting a vapor MTBE-containing stream over a fixed bed of acid cation exchange resin and separating isobutene from methanol and unreacted MTBE by distillation columns.